1. Field of the Invention
The present invention relates to a drive apparatus, and more particularly, to a drive apparatus having a shortened axial dimension.
2. Description of the Related Art
Conventionally, stepping motors have been widely used as driving sources for various apparatuses. As a first exemplified prior art, there has been proposed a stepping motor which is small in diameter around a rotating shaft of the motor and has a high output (see Japanese Laid-Open Patent Publication No. 09-331666, for example).
FIG. 21 is an exploded perspective view showing components of the stepping motor according to the first exemplified prior art, and FIG. 22 is a sectional view showing the axial construction of the stepping motor in an assembled state.
In FIGS. 21 and 22, the stepping motor includes a magnet 101, a first coil 102, a second coil 103, a first stator 104, a second stator 105, an output shaft 106, and a coupling ring 107. The magnet 101 is formed into a cylindrical shape and circumferentially divided into four pieces, which are alternately magnetized to different polarities.
In the stepping motor having the above described construction, when electric power is supplied to the first coil 102 which is thereby energized, a first magnetic circuit is formed that passes through a first outer magnetic pole 104A, a first inner magnetic pole 104C, and the magnet 101. When the second coil 103 is energized, a second magnetic circuit is formed that passes through a second outer magnetic pole 105A, a second inner magnetic pole 105C, and the magnet 101. By changing magnetic fluxes flowing though the first and second magnetic circuits, a force acting on the magnet 101 is caused to change, whereby a rotor formed by the magnet 101 and the output shaft 106 is rotated.
In the stepping motor according to the first exemplified prior art, gaps between the first and second magnetic circuits are present only between the outer magnetic poles and the magnet 101 and between the magnet 101 and the inner magnetic poles, and therefore, the magnetic circuits have a reduced magnetic resistance. Furthermore, fluxes efficiently act on the magnet 101 between the outer and inner magnetic poles. As a result, stronger magnetic fluxes can be generated with less electric current, whereby the output of the stepping motor can be increased.
A stepping motor according to a second exemplified prior art is formed into a hollow cylindrical shape while having magnetic circuits that have the same construction as those of the first exemplified prior art (see, Japanese Laid-open Patent Publication No. 2002-51526, for example). When mounted on a camera, the stepping motor of this type is disposed parallel to the optical axis in a lens barrel of the camera, with a diaphragm blade, a shutter, lenses, and the like disposed on the inner-diameter side of the stepping motor. As a result, the camera has a construction that is much efficient in space utilization than in a motor of a solid construction provided with no spaces on the inner-diameter side of the motor, making it possible to decrease the diameter of the lens barrel of the camera.
The above described stepping motors according to the first and second exemplified prior arts each include a rotor formed by a permanent magnet which is large in diameter. As a result, the moment of inertia of the rotor is large, thus entailing a drawback that the response characteristic of the stepping motor is deteriorated at high-speed rotation.
To improve the response characteristic at high-speed rotation, there has been proposed a stepping motor whose stator is formed by a coil and a permanent magnet and whose rotor is formed by a yoke (see, for example, Japanese Laid-open Patent Publication No. 2005-204453).
FIG. 23 is an exploded perspective view showing components of a stepping motor according to a third exemplified prior art, and FIG. 24 is a sectional view showing the axial construction of the stepping motor in an assembled state.
Referring to FIGS. 23 and 24, the stepping motor includes a first outside yoke 207, a first inside yoke 208, a first top panel yoke 209, a second outside yoke 210, a second inside yoke 211, and a second top panel yoke 212. The stepping motor further includes a first coil 213, a second coil 214, a first magnet 215, a second magnet 216, a first rotor yoke 217, and a second rotor yoke 218.
Magnetic fluxes generated by the energized first and second coils 213, 214 flow through first and second magnetic circuits, each of which passes through the first or second outside yoke, the first or second top panel yoke, the first or second inside yoke, the first or second rotor yoke, and the first or second magnet. By changing the magnetic fluxes flowing though the first and second magnetic circuits, forces acting between the magnets and the first and second rotor yokes are caused to change, whereby a rotor formed by these rotor yokes is rotated.
With the stepping motor according to the third exemplified prior art, rotor yokes smaller in the moment of inertia than a magnet can be disposed as a rotor, while permitting the motor to have the same or similar magnetic path construction to that of the first exemplified prior art stepping motor. As a result, the response characteristic of the motor at high-speed rotation can be improved.
However, in each of the first to third exemplified stepping motors, the first and second magnetic circuits are juxtaposed to each other in the axial direction. In order to reduce the axial direction of this type of stepping motor, the distance between the first and second magnetic circuits must be decreased.
In the construction where the distance between the first and second magnetic circuits is made small, interference occurs between the magnetic circuits, causing problems that the stepping motor cannot stop at a predetermined rotary position and a cogging torque increases. To obviate this, some distance is required between the first and second magnetic circuits, making it difficult to decrease the axial dimension of the stepping motor, which causes a problem.